A prior copending and commonly assigned United States patent application of Hecht et al., which was filed Dec. 22, 1993 under Ser. No. 08/172,508 on "Global Addressability of Self-Clocking Glyph Codes" (D/93689) describes address codes for specifying the relative addresses of the synchronization glyphs within a self-clocking glyph code pattern. This inventions builds on and extends this relative addressing concept. As such, the present invention is applicable to embedded data blocks in general, even though it is described hereinbelow in the context of self-clocking glyph code patterns.
As is known, self-clocking glyph codes are suitable for embedding machine readable data in images of various descriptions, but they are especially advantaged for applications that require or benefit from having the machine readable data embedded in the images in a visually unobtrusive or esthetically pleasing way. Specifically, to provide a visually homogeneous glyph pattern (i.e., a glyph pattern in which the glyphs are substantially indistinguishable from each other when viewed by the unaided eye under normal lighting conditions), all of the glyphs of a self-clocking glyph code typically are defined by symbols from the same symbol set, such as slash-like symbols that are tilted from vertical at approximately +45.degree. and -45.degree. to encode binary "0's" and "1's", respectively.
More generally, however, it is to be understood that a "glyph" is an "embedded data character," which is defined as being a two dimensional image symbology that has at least two graphical states for encoding the logical states ("1" and "0") of a single bit. An "embedded data block" (EDB) in turn, is a two dimensional image symbology for the storage and retrieval of data. EDBs are composed of embedded data characters; some of which are encoded to define a synchronization frame and others of which are encoded to carry user/application-specific information. A "glyph pattern" is an instance of an EDB.